Missing 3D printing category tab in tool shelf after activating the addon
The panel for the 3D printing tools is only visible when the active object is a mesh object. The active object is the one highlighted in a slightly lighter colour (depending on the theme used).

There are times when an active object can be deleted or deselected without a new object being set as active, usually this happens using the outliner or a script.

Change your selection or &#8679; Shift select one of your selected mesh objects to make it the active one so that the panel shows up.